The present invention relates generally to apparatus and methods of sealing a rotating shaft in a fluid conveying system and more particularly to the sealing of the driving shaft of a conventional gear pump and like fluid conveying apparatus.
In any fluid conveying system and in substantially all fluid pumps, an engineering problem exists in the sealing of the rotating shaft to which the impeller is mounted at the location at which the shaft extends outwardly through a wall of the system separating its interior from the outside. This problem is particularly acute in pumps such as gear pumps which are operable at either or both very high internal pressures or at relatively low, vacuum-like internal pressures relative to the atmosphereic pressure of the ambient area. As is well known, gear pumps are essentially positive displacement pumps which have widely ranging applicability for many various fluid conveying uses and applications. For example, gear pumps have in recent years become increasingly utilized widely in the textile and similar fields for conveying polymeric and other similar viscous material from one process location, e.g. a reactor vessel such as a heat exchanger, to another process location, e.g. a downstream extruding system, and, in such varied applications, gear pumps may be operated either at relatively high or relatively low internal pressures depending upon the application. Conventionally, a gear pump basically includes a substantially enclosed housing in which a pair of meshing gear are mounted with the driving gear supported on a rotating shaft extending outwardly through the housing to be connected to the output shaft of a driving motor. In the past, a substantial problem has existed in providing an effective seal for the gear pump shaft which, on the one hand, would prevent leakage outwardly from the gear pump housing when operated under high internal pressures and, on the other hand, would prevent intake of ambient air when operated at vacuum or other relatively low internal pressures. The leakage of the conveyed working fluid is at best inefficient and costly and at worst renders the system effectively inoperable and may result in permanent damage to the gear pump. Similarly, the intake of ambient air at best produces air bubbles in the conveyed working fluid which ultimately results in an inferior product and at worst renders the system essentially inoperable.
In the past, packing material usually consisting of a graphite-impregnated cotton material has been squeezed about the gear pump shaft at the location it extends through its housing in order to seal it. Disadvantageously, such packing material produces substantial frictional contact between the material and the shaft which increases the required driving power for the system and produces excessive wear of the packing material necessitating its frequent replacement, all of which substantially increases the cost of the gear pump operation. In addition, the frictional forces created by the packing material have been known to cause wearing of the shaft ultimately requiring its replacement as well. Furthermore, such packing material is often relatively ineffective for its intended sealing purpose under conditions of substantial internal and external pressure differentials created in many conventional gear pump applications.
One proposed solution to the foregoing problems is set forth in Fox U.S. Pat. No. 4,336,213 wherein is disclosed a sealing sleeve mounted to the gear pump housing to receive its rotating shaft. The interior of the sealing sleeve is provided with a helical channel of a hand relative to the direction of rotation of the gear pump shaft to cause the working fluid entering the channel area about the shaft to be directed back into the housing and thereby, in theory, to seal the shaft effectively against fluid leakage in a frictionless, non-wearing manner.
However, in practice, the sealing sleeve of the Fox patent has been found to provide a generally ineffective seal in many ordinary gear pump applications. As will be understood, the internal helical channel of the sealing sleeve merely provides some resistance to outward flow of the working fluid about the shaft. In highly pressurized gear pump applications, the resistance provided by the sealing sleeve is insufficient to entirely prevent fluid leakage, particularly when the working fluid is of relatively low viscosity in which cases fluid leakage is sometimes so substantial that the gear pump cannot be effectively operated. On the other hand, in applications in which the gear pump is operated at low, vacuum-like internal pressures, the sealing sleeve is generally ineffective to preven air bubbles from being drawn into the gear pump housing through the channel area.